The world has just SIX species of tigers left in the wild: Ecologists finally agree on the number of big cats that still roam free

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There are six types of tiger still remaining, ecologists have concluded, in a study which could boost efforts to save the big cat from extinction.

There are fewer than 4,000 tigers left in the wild and efforts to protect them have been hindered by disagreement over the number of subspecies. Experts have deliberated over whether there were two, five or six types of the beasts still in existence. But a groundbreaking study has revealed the six subspecies that still roam the wild: the Bengal tiger, Amur tiger, South China tiger, Sumatran tiger, Indochinese tiger and Malayan tiger. Three other subspecies of the stripy cat have already been wiped out, disappearing at least three decades ago. Scientists also found evidence that these subspecies each have a unique evolutionary history, by looking at highly structured patterns across the groups.

The findings were confirmed after researchers analysed the complete set of genes of 32 tigers and found they fell into six genetically distinct groups. The screening method was also the only way they could look for signals that different groups of tigers had undergone natural selection to adapt to the environments found in the unique geographic regions where they live. The new findings provide the strongest genetic evidence yet for subspecies delineation in tigers.

Fossil evidence shows that tigers go back two to three million years. But the evidence from their genes shows that all surviving tigers only trace back about 110,000 years, when the predators suffered a historic population bottleneck. The strongest signal of selection researchers found was in the Sumatran tiger, across a genomic region that contains the body-size-related ADH7 gene. The researchers suggest that the Sumatran tiger might have been selected for smaller size to reduce its energy demands, allowing it to survive on the island's smaller prey animals, such as wild pigs and muntjac, a small deer.

Dr Shu-Jin Luo, from Peking University in China, said: 'The lack of consensus over the number of tiger subspecies has partially hindered the global effort to recover the species from the brink of extinction.This is because both captive breeding and landscape intervention of wild populations increasingly requires an explicit delineation of the conservation management units. This study is the first to reveal the tiger's natural history from a whole-genomic perspective. It provides robust, genome-wide evidence for the origin and evolution of this charismatic megafauna species. Tigers are not all alike. Tigers from Russia are evolutionarily distinct from those from India. Even tigers from Malaysia and Indonesia are different.'

Dr Luo said: 'In the end, we were quite amazed that, by performing a stepwise. There is very little gene flow among tiger populations.

Study lead author Yue-Chen Liu said: 'That's quite unique among the big cats - nothing that several other species, such as the jaguar, have shown much more evidence of intermixing across whole continents.'

The origin of the South China tiger remains unresolved since only one tiger in captivity was used in the study, after the subspecies became extinct in the wild. The researchers plan to study old specimens with known origin from all over China to fill in the missing pieces of living tigers' evolutionary history. They are also retrieving information from genes of historical specimens, including those representing the extinct Caspian, Javan, and Bali tigers.